Advance ceramic fibers are needed for use as reinforcement for ceramic and metal matrix composites. The fiber reinforcement enhances toughness, strength, stiffness and creep resistance for both ceramic and metal matrix composites. Silicon carbide fiber has long been sought as the potential reinforcement because it is predicted to possess high elastic modulus, excellent thermal stability, attractive thermal expansion characteristics, and relatively low density. Often the desired temperature exceeds the fiber capability, i.e. the fiber strength degrades significantly in just a few hours. For example Nicalon.TM. (SiCO) and Tyranno.TM. (SiCOTi) silicon oxycarbide fibers degrade at temperatures above 1200.degree. C.
Several methods have been developed to make silicon carbide (SiC) based continuous fibers. One approach consists of depositing SiC by CVD on a core of carbon monofilament. However, because of the large diameter of these fibers, they are poorly suited for ceramic matrix composite (CMC) reinforcement and are too stiff to be woven into fabric as required by CMC applications. Another approach involves the melt-spinning of organosilicon polymers to form precursor fibers. The precursor fibers are then pyrolytically transformed into ceramic fibers. The final products have not been pure SiC fiber. This process is used to produce the Nicalon.TM. and Tyranno.TM. fibers discussed above.
A third method, disclosed in U.S. Pat. No. 5,279,780 to Lipowitz et al. produces near-stoichiometric polycrystalline silicon carbide (SiC) fibers that can withstand temperatures to 1400.degree. C. while still maintaining reasonable strength. The process comprises heating an amorphous or microcrystalline ceramic fiber containing silicon, carbon and oxygen in an environment comprising a volatile sintering aid. According to U.S. Pat. No. 5,279,780 the sintering aid may be used by merely introducing it into the environment for pyrolysis in its volatile state or it may be diluted in an inert carrier gas such as argon or helium. The process may be run batch or continuous. When this process is run continuous it is still limited to slow line rates due to the time that is needed for the boron to diffuse into the fibers.
It has now been found that when carbon monoxide is present in the reactor with the purge gas, high rates of production can be achieved while still obtaining fibers with dense microstructure and with good physical properties such as tensile strength and modulus.
It is therefore an object of this invention to provide a continuous method for the production of polycrystalline ceramic fibers from silicon oxycarbide (SiCO) ceramic fibers.